In designing a magnetic recording disc, there are a number of factors, depending upon the properties of available recording heads, that must be considered and provided for. Among the magnetic properties that are desirable, especially where high magnetic recording density is an object, is a high coercivity limited only by the write capability of the recording head. The recording system also requires that the head signal output of the recording head be greater than some minimum value. It is required in turn for thin recording films that the product of the magnetic recording film thickness and its remanent mangetization (hereinafter referred to as the magnetization-thickness product) be greater than some corresponding minimum value. In addition, the high coercivity and desired magnetization-thickness product should be accompanied by a high degree of squareness of the M-H hysteresis loop of the magnetic film.
With the given values of coercivity and magnetization-thickness product established, the recording density is desirably the maximum value that is consistent with the minimum value of resolution for the recording process. Because resolution generally decreases both with increasing recording density and with an increasing value of the ratio of the magnetization-thickness product to coercivity, optimization of the recording system requires that the disc coercivity and its magnetization-thickness product be essentially the corresponding maximum and minimum values established above. Any value of magnetization-thickness product greater than its minimum value and any value of coercivity less than its maximum value necessarily results in a recording density less than what would otherwise be achievable with the recording head.
Hence, there exists a need to vary the coercivity and magnetization-thickness product independently. Further, the values of these parameters need to change as improved recording heads become available and there is, therefore, an additional need for independent variability of coercivity and magnetization-thickness product over a rather large range of values.
Another important constraint is that the necessary independent variability of magnetic properties be achievable on smooth substrates which are suitable for use as the mechanical support for the recording disc. It is also desirable that once a process to achieve targeted or predetermined magnetic properties is in place, minor independent adjustments to the magnetic properties can be readily made by means of relatively easy and straight forward changes in process parameters.
U.S. Pat. No. 4,610,911, issued to James E. Opfer et al and assigned to the same assignee as the present appication, provides a solution to the foregoing needs. In that patent, a magnetic layer of an alloy of cobalt and platinum (about 1 to 20 at% Pt) is deposited on a chromium underlayer. The platinum content in the magnetic film is varied so as to provide a magnetic film with a desired coercivity which is significantly less dependent upon film thickness. This permits the cobalt-platinum film thickness to be adjusted to meet a wide range of recording system design parameters. The coercivity of the magnetic film may be determined primarily by the amount of platinum in the alloy and the saturation magnetization-thickness product of the film is determined primarily by the film thickness.
U.S. Pat. No. 4,652,499 is directed to a magnetic recording medium with a chromium alloy underlayer and a cobalt-based magnetic layer. In particular, an improved cobalt-platinum (CoPt) thin film metal alloy media for longitudinal magnetic recording has a squareness greater than prior CoPt thin metal alloy media. An underlayer of a body-centered-cubic (BCC) chromium-based alloy with a lattice constant greater than chromium, such as chromium-vanadium, is formed between the substrate and the CoPt magnetic layer. The underlayer also improves the magnetic properties of the media when the magnetic layer is an alloy of cobalt-platinum-chromium (CoPtCr). A comparison of magnetic properties of (Co.sub.85 Pt.sub.15).sub.93.5 Cr.sub.6.5 on various substrates (Cr; Cr.sub.80 V.sub.20) is shown. A slight improvement using the CrV underlayer is noted. Specifically, a coercivity of 1420 Oe is listed, along with a magnetization-thickness product of 2.75.times.10.sup.-3 emu/cm.sup.2, a squareness ratio (M.sub.r /M.sub.s) of 0.90, and a coercivity squareness ratio (S*) of 0.906. However, this composition is not expected to provide good corrosion resistance and reduced noise.
U.S. Pat. No. 4,654,276 is directed to a magnetic recording medium with an underlayer and a cobalt-based magnetic layer. In this patent, a tungsten (W) underlayer is used to increase the coercivity of CoPt or CoPtCr magnetic layers, especially where the tungsten underlayer and the magnetic layer are deposited so as to form an intermetallic compound, Co.sub.3 W, at the interface between the two layers. In this patent, the chromium content in the CoPtCr alloy is 20 at% and the platinum content is 8 at%. A film 1000 A thick evidenced a coercivity of 970 Oe, a coercivity squareness (S*) of 0.89, and a remanence-thickness product of 1.68.times.10.sup.-3 emu/cm.sup.2. However, tungsten presents manufacturability problems, due to poor adhesion resulting from high stress. Further, the coercivity of the combination is about 50% less than that obtained with a chromium underlayer for a given Co-Pt-Cr composition and thickness.
Japanese Application No. 198568/82 (Laid-Open No. 88806/84), laid-open May 22, 1984, is directed to a magnetic memory comprising a magnetic alloy of CoPtCr or CoPtTa on a non-magnetic underlayer of a nickel-phosphorus alloy or aluminum oxide. The chromium content may range from about 1 to 17 at%, while the platinum content ranges from about 9 to 35 at%. A coercivity of 500 to 1200 Oe is obtained, along with a squareness ratio (B.sub.r /B.sub.s) of 0.7 to 0.9, a coercivity squareness (S*) of 0.7 to 0.9. However, higher platinum concentrations require greater thickness of the magnetic layer to achieve reasonable magnetic recording properties.
The foregoing latter three references lack one or more of the desirable properties needed for improved magnetic recording media, including such attributes as magnetic properties, recording performance, corrosion resistance, and manufacturability constraints. Further, the thickness of magnetic layers used in magnetic recording needs to be kept as low as possible to provide a strong head field gradient, which will yield a sharper magnetic transition, an aspect not addressed by the three references. Accordingly, a need remains to develop a magnetic recording medium having the requisite characteristics described above, employing convenient processing conditions.